Scope issues with populating array from multiple mongodb calls

Question

I have a game grid that I'm trying to fit into an array of rows each with a number of blocks. The blocks are stored in the same collection and have row and column fields.

My current attempt is as follows.

exports.findById = function(req,res) {
    var id = req.params.id;
    console.log('Retrieving game #' + id);
    db.games.findOne( { "_id" : ObjectId(id) }, function(err, game_obj) {
        if(err || !game_obj) {
            console.log("no game");
            res.send({'error':'An error has occurred'});
        } else {
            var rows = [];
            function getRowBlocks(err,blocks) {
                if(err || !blocks) {
                    console.log("no blocks");
                    res.send({'error':'An error has occurred'});
                } else {                    
                    console.log(blocks[0].row + " has " + blocks.length + " blocks");                   
                    rows[blocks[0].row] = blocks;
                }
            }
            for(i=0;i<game_obj.farthest_row;i++) {  
                db.blocks.find( { "game_id" : ObjectId(id), "row" : i     }).sort( { "column" : 1 }).toArray(getRowBlocks);
            }
            res.send('game', { row_list : rows, game : game_obj});
        }
    });
}

But due to the scope of the mongodb find operation called for each row I cannot actually get the blocks stored in the rows variable. And because there are multiple operations run I cannot simple put the res.send... inside the toArray callback.

I could only use one query and build the array inside the callback and then pass that on, but I think this would be a lot less efficient considering that the multiple queries are happening asynchronously and that there could be a large number of rows.

I think I could do it if I could pass the rows array into the query callback, but I haven't been able to find out if that's possible.

robertklep · Accepted Answer · 2013-05-23 18:28:51Z

1

That's what async.parallel is for:

// build a list of query-functions
var queries = [];
for (var i = 0; i < game_obj.farthest_row ; i++) {
  queries.push(function(i, cb) {
    db.blocks
      .find( { "game_id" : ObjectId(id), "row" : i })
      .sort( { "column" : 1 })
      .toArray(cb);
  }.bind(this, i)); // create a partial function to create a newly scoped 'i'
}

// pass the query functions to async, which will run them in parallel,
// gather all the results, and finally call the callback function with
// the results-array
async.parallel(queries, function(err, results) {
  if (err)
    // handle error

  // results now contains the query results (array-in-array)
  results.forEach(function(blocks) {
    getRowBlocks(null, blocks); // this probably requires a rewrite...
  });
});

answered May 23, 2013 at 18:28

robertklep

205k38 gold badges421 silver badges413 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Scope issues with populating array from multiple mongodb calls

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related